One of the problems that face platform commanders on a modern land battlefield is to positively identify targets which are detected within range of their weapon systems. Detection is not only based on visual means, such as panoramic or telescopic sights, but is also considerably enhanced by using thermal imaging equipment. However, even with the most sophisticated thermal viewers, identification of land vehicles is not straightforward. The signatures of land vehicles detected by those types of thermal viewers are dependent, to a very large degree, on uncontrollable factors such as the time a vehicle's engine has been running, the time a vehicle has been exposed to direct sunlight, etc. Identification of friend-or-foe (IFF) presents a very difficult decision for a tank commander who must often decide in a split-second whether or not to engage a detected target while, at the same time, trying to minimize any possibility of fratricide killing.
No current systems exists which provide reliable, rapid and positive identification of friend-or-foe (IFF) vehicles on modern land battlefields. Commanders still rely on low-resolution visual and infrared images to determine if detected targets, be they tanks or other support vehicles, are enemy ones or not. That information may be possibly supported by information derived from a radio network. However, it is not always possible to obtain information from a radio since tank commanders often have to operate under radio silence in order to avoid being detected by the enemy.
A few techniques of IFF are known with one way to achieve an IFF function being for a vehicle, such as a tank, to carry a transponder that emits a coded return when a radar pulse is received by its receiver. This type of system is described in U.S. Pat. No. 4,851,849 by Otto Albersdoerfer. A similar type of system is described in U.S. Pat. No. 4,694,297 by Alan Sewards. However, the system in the U.S. Pat. No. 4,694,297 does not require an active transponder but only an antenna on a target vehicle which can re-radiate or reflect a radar beam and modulate that re-radiated beam in a distinctive manner. This system is based on the idea that an illuminating radar would detect only a small reflected signal from a good antenna which is terminated in a matched load. However, all of the energy intercepted by that antenna will be re-radiated when the antenna terminating impedance provides a short circuit. A substantial reflected signal is then created which may be detected by the source of the illuminating radar beam. Therefore, an antenna on a target vehicle with a variable terminating impedance can modulate a re-radiated radar beam to the source and provide an identification signal with the passively reflected beam.
A few other techniques (mainly Optical IFF) are known such as those described in German Patents 2,215,463 (Precitronic Feinmech); 2,142,944 (Precintronic Ges Feinmech); 3,323,698 (Ant. Machr Ichtentech) and 3,113,154 (Precitronic Ges FEI). IFF systems are also described in French Patent 2,605,416 by Joquiet J. C. and U.S. Pat. No. 4,814,769 by Leon Robin et al.